Figure 9.

Elastic fibers (black arrows) appeared first in the anterior leaflet before being deposited within the chordae tendineae. Representative images demonstrating the presence of elastic fibers near the fixed edge in Movat-Pentachrome–stained mitral anterior leaflet (A,C,E,G) and strut chordae tendineae (B,D,F,H) throughout gestation. (A,B) Valve in early second trimester (111 days gestation), (C,D) third trimester (212 days gestation), (E,F) third trimester (219 days gestation), and (G,H) full term (270 days). Collagen is stained yellow-orange, elastic fibers dark purple, and cell nuclei dark red-purple. Elastic fibers first appeared in the leaflet in early second trimester (A) near the fixed edge. Elastic fibers accumulated throughout gestation, extending into the rest of the leaflet (C,E,G). Elastic fibers were not present in the chordae during second trimester (B), appearing only sparsely at the outer edges of the chordae in early third trimester (D) then increasing in density and length until full term (F,H). By late gestation, elastic fibers were also present within the bulk of the chordae (H).